Thermodynamic Cycles of Engines Questions and Answers

Answer: A

Combustion in a spark ignition engine is fast enough that the piston barely moves while it happens, so constant volume is a reasonable idealisation. The Diesel cycle by contrast adds heat at constant pressure, which reflects the slower burning of injected fuel as the piston descends.

Answer: B

Fuel is injected progressively as the piston descends, so the pressure stays roughly constant while the volume increases. That difference from the Otto cycle is what introduces the cut-off ratio as a second parameter alongside the compression ratio.

Answer: C

Real combustion begins rapidly near top dead centre and continues as the piston descends, so neither idealisation alone fits. Splitting the heat addition between the two processes gives a closer match to the indicator diagram an actual engine produces.

Answer: D

The relation is \( 1 - r^{1-\gamma} \), so efficiency rises with compression ratio and with the specific heat ratio of the working fluid. Notably the quantity of heat added does not appear, which means the ideal efficiency is independent of load.

Answer: A

Constant volume heat addition occurs at a higher average temperature than constant pressure addition does. Real diesel engines nevertheless outperform petrol engines because they run at far higher compression ratios, which more than recovers the theoretical deficit.

Answer: B

A larger cut-off ratio means heat is added over a greater part of the expansion stroke, which lowers the cycle efficiency. Efficiency therefore falls as the engine is loaded more heavily, which is a genuine difference from the ideal Otto cycle.

Answer: C

It consists of isentropic compression, constant pressure heat addition, isentropic expansion and constant pressure heat rejection. The continuous flow through separate components is what distinguishes it from the reciprocating cycles that use one chamber for every process.

Answer: D

The relation is \( 1 - r_p^{(1-\gamma)/\gamma} \), so raising the pressure ratio always improves the ideal efficiency. Turbine entry temperature does not appear at all, although it governs the specific work and dominates the real cycle.

Answer: A

The compressor typically absorbs more than half of what the turbine produces, so the net output is a modest difference between two large numbers. That is why small losses in either component have a disproportionate effect on the useful work available.

Answer: B

Compressor work grows faster than turbine work once the pressure ratio is high, so the net work per kilogram falls. Ideal efficiency keeps rising, which is why the two criteria give different optimum pressure ratios and a designer must choose between them.